1. Technical Field
The invention relates to optical waveguides and photonic circuits. In particular, the invention relates to optical waveguide resonators used in photonic systems.
2. Description of Related Art
An optical ring resonator generally comprises a segment or segments of optical waveguide arranged in a ring-shape or essentially a closed loop. Such ring resonators have a variety of important uses in photonic systems. For example, the ring resonator may be part of a photonic filter. In another example, the ring resonator may be employed to one or more of generate, modulate or amplify an optical signal by including an active element or elements along the optical waveguide segment(s) within the ring resonator. A pumped laser may be realized using a ring resonator, for example.
An important characteristic of resonators including, but not limited to, the ring resonator, is optical loss. Optical loss results in a general loss of optical signal energy within the resonator. Loss of the optical signal energy degrades a performance of the resonator and may limit its utility in some applications. A measure of optical loss is a so-called quality factor or ‘Q’ of the resonator. Good resonators having low loss generally exhibit a high Q and are referred to as ‘high-Q resonators’ as a result.
In part, optical loss in a ring resonator may be due in part to waveguide loss. Among the sources of waveguide loss are material loss and scattering loss. Material loss occurs when the optical signal is either absorbed or scattered by a material of the optical waveguide. On the other hand, scattering loss is caused by a destructive interaction between the optical signal and a physical boundary of another physical structure of the optical waveguide. Scattering loss generally impedes propagation of the optical signal. Tight coupling between the optical signal and the optical waveguide may further exacerbate such scattering loss. Material loss and scattering loss are generally related to a material make-up and physical type (e.g., cross sectional shape) of the optical waveguide being employed.
Optical loss in a ring resonator may also be associated with or due to the ring-shape of the ring resonator. In particular, ring resonators often experience waveguide loss characterized as leakage loss. In leakage loss, optical loss results when some of the optical signal within the ring resonator escapes or radiates out of the resonator as the optical signal is guided around the closed ring-shaped path of the ring resonator. In particular, the ring-shape generally requires that an optical signal being guided within the resonator must change direction. This change in direction can and does lead to some optical loss, especially when the optical signal is not tightly coupled to the optical waveguide.
Slab optical waveguides such as, but not limited to, ridge-loaded waveguides and inverse ridge-loaded waveguides may exhibit inherently lower scattering loss since the optical signal is loosely coupled to the waveguide. However, such slab optical waveguides may exacerbate the leakage loss especially in ring resonators having one or both of small dimensions and tight turns within the optical waveguide of the ring resonator.